Advanced semiconductor chips employ multiple types of field effect transistors having different threshold voltages, on-current per unit width, and off-current per unit length. Field effect transistors having a high threshold voltage are typically called “low power” devices, which have a low on-current and a low off-current. Field effect transistors having a low threshold voltage are called “high performance” devices, which has a high on-current and a high off-current. By employing a mixture of low power devices and high performance devices, a semiconductor chip may provide optimal performance at an optimal power consumption level.
Devices having different threshold voltages may be obtained by varying dopant concentration of a doped semiconductor well in which the body of a field effect transistor is formed for each value of the threshold voltages. Thus, a high performance device employs a doped semiconductor well having a low dopant concentration, while a low power device employs another doped semiconductor well having a high dopant concentration. For each setting of well doping, a dedicated implantation mask is employed during a corresponding ion implantation step for formation of a doped semiconductor well, thereby increasing processing complexity and cost of manufacturing.
In addition to the increased processing cost, low power devices employing a doped semiconductor well having a high dopant concentration also suffer from increased junction leakage. Typically, low power devices display a value for reverse junction leakage current to forward junction leakage current ratio that is many orders of magnitude higher than a corresponding value for high performance devices. As the total leakage current is dominated by a junction leakage current, the method of providing a low power device by increasing the dopant concentration of a doped semiconductor well becomes more and more difficult. Furthermore, in the case where such FETs are SOI FETs and have so-called floating bodies, the increased junction leakage of the low power devices tends to increase the forward-bias voltage of the floating body, thereby lowering Vt and opposing the desired effect of lower leakage currents.
In summary, a high dopant concentration in a doped semiconductor well containing the body of a transistor induces a high junction leakage current. Due to the increase in the junction leakage current, the method of modulating the well doping to form a low leakage field effect transistor becomes less effective in suppressing the total leakage current despite a high threshold voltage.